I do agree there will be some bad eggs amongst the membership, but, given the present law, there wil be a greater number of genuine victims of UK legislation there, too. If the system worked the other way round, and this was a 'mothers for justice' group, included in the membership would no doubt be some hideously awful, abusive mothers trying to manipulate for their own ends. You're right. it can work both ways.
bear in mind that, according to social services figures covering all recognised forms of abuse, overall mothers are more likely to abuse their kids than fathers (and don't worry, there's plenty of room for discussion on those figures on their own), I don't think that the 'some of those guys are abusive' claim can go too far to destroy F4Js case, really.
You've mentioned that fathers can be denied contact for good reason, and also fathers can be denied contact without good reason. The fact that latter exist, + the state allows this through its laws, is surely good enough reason, don't you think, for someone to challenge this. There's plenty of groups of people in this country who can be pissed on by the state without anybody representing them. The unemployed, for example, have little in the way of defence or representation. Parents who are denied contact with tgheir kids for no good reason surely deserve a voice, too.
I think one of the 'celebrity' figures behind this movement is Bob Geldof. If I'm right here (not sure I am yet), then regardless of what you think of him as a person, what kind of father do you think he is?